title: Technical note: Nitrogen isotopic fractionation can be used to predict nitrogen-use efficiency in dairy cows fed temperate pasture.
abstract: The objective of this study was to investigate the relationship between nitrogen isotopic fractionation (δ(15)N) and nitrogen-use efficiency (milk nitrogen/nitrogen intake; NUE) in pasture-fed dairy cows supplemented with increasing levels of urea to mimic high rumen degradable protein pastures in spring. Fifteen cows were randomly assigned to freshly cut pasture and either supplemented with 0, 250, or 336 g urea/d. Feed, milk, and plasma were analyzed for δ(15)N, milk and plasma for urea nitrogen concentration, and plasma for ammonia concentration. Treatment effects were tested using ANOVA and relationships between variables were established by linear regression. Lower dry matter intake (P = 0.002) and milk yield (P = 0.002) occurred with the highest urea supplementation (336 g urea/d) compared with the other two treatments. There was a strong linear relationship between milk δ(15)N - feed δ(15)N and NUE: [NUE (%) = 58.9 - 10.17 × milk δ(15)N - feed δ(15)N (‰) (r(2) = 0.83, P < 0.001, SE = 1.67)] and between plasma δ(15)N - feed δ(15)N and NUE: [NUE (%) = 52.4 - 8.61 × plasma δ(15)N - feed δ(15)N (‰) (r(2) = 0.85, P < 0.001, SE = 1.56)] . This study confirmed the potential use of δ(15)N to predict NUE in cows consuming different levels of rumen degradable protein.
authors.affiliation: Faculty of Agriculture and Life Sciences, P.O. Box 84, Lincoln University, New Zealand.
authors.forename: L
authors.initials: L
authors.lastname: Cheng
authors.forename: A J
authors.initials: AJ
authors.lastname: Sheahan
authors.forename: S J
authors.initials: SJ
authors.lastname: Gibbs
authors.forename: A G
authors.initials: AG
authors.lastname: Rius
authors.forename: J K
authors.initials: JK
authors.lastname: Kay
authors.forename: S
authors.initials: S
authors.lastname: Meier
authors.forename: G R
authors.initials: GR
authors.lastname: Edwards
authors.forename: R J
authors.initials: RJ
authors.lastname: Dewhurst
authors.forename: J R
authors.initials: JR
authors.lastname: Roche
mesh_terms: D000821:Animal Feed / Q000032:analysis*; D000822:Animal Husbandry; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D002417:Cattle / Q000378:metabolism*; D002980:Climate; D004032:Diet / Q000662:veterinary*; D019587:Dietary Supplements; D005260:Female; D007774:Lactation / Q000502:physiology; D008892:Milk / Q000737:chemistry / Q000378:metabolism; D009584:Nitrogen / Q000737:chemistry* / Q000378:metabolism*; D014508:Urea / Q000494:pharmacology
chemical_list: D014508:Urea; D009584:Nitrogen